Blades used in the turbine section of gas turbine engines are exposed to combustion gases, high mechanical force, and foreign object impact. This, coupled with the high operating temperature, create high levels of stress in the blade. Blade tips including blade tip shelves (an end piece of the airfoil) and blade squealers (elevated material surrounding the blade tip), blade airfoil sections, and the blade platform are particularly susceptible to stress related damages, including areas of wear and cracks. These cracks may extend from the tip of the airfoil downward toward the platform, sometimes extending past the blade shelf (also known as tip cap) adjacent the blade tip.
It is known to replace worn or cracked blade squealers with non-structural replacement material. This replacement material is considered non-structural primarily because the stresses are relatively low in this location, and as a result, consequences of damage are relatively minimal in terms of performance. Unfortunately, cracking is very often found below (toward the platform) the tip shelf, extending into the airfoil body. For example, the cracks may extend 30 mm below the blade tip. Replacement of this material (below the squealer) is more difficult and must be considered to be of a more structural requirement, wherein certain minimum mechanical properties must be attained in order to sustain the greater stresses encountered in the airfoil body.
For the most difficult to weld superalloys, there is no known process to replace such extensive portions of a turbine blade. Grinding out and re-welding cracks using a hot box to maximize material ductility during the process has met with limited success. Cutting off the entire distressed blade tip and welding is not possible for at least two reasons. First, the material itself does not accommodate butt welding. It would crack due to shrinkage stresses and high restraint. Second, ribs disposed within the airfoil (serving structural function and cooling air management) could not be accessed for butt welding. Consequently, there is room in the art for improved methods of building and/or repairing blade airfoils.